Assessed June 2010 by COSSARO as Endangered

Part 1: COSSARO Candidate Species at Risk Evaluation Form – June 2010

Fowler’s Toad (Anaxyrus fowleri)

Current designations:

GRANKG5 April 2002. NatureServe
NRANK Canada – N2 Oct 1998 NatureServe
COSEWIC – Endangered April 2010
SARA – Threatened (Schedule 1) (based on COSEWIC 2000)
General Status Canada – At Risk 2005
ESA 2007 – Threatened 2000
SRANKS2 NatureServe General Status Ontario – At Risk

Distribution and status outside Ontario:

Fowler’s Toad inhabits much of North America east of the Great Plains and south of the Great Lakes (Green 2005) except for the southern Atlantic coastal plain from North Carolina to northern Florida. On the western edge of the range, the larger Woodhouse’s Toad (A. woodhousii) intergrades with Fowler’s Toad along a line south from Lake Michigan to the Gulf of Mexico at the Texas/ Louisiana border (Meacham 1962). Fowler’s Toad extends north to the west side of Michigan’s Lower Peninsula, reaching almost to the Mackinac Strait, to the shores of Lake Erie in Ohio, Ontario and northern Pennsylvania and through New Jersey, southeast New York and southern Vermont and New Hampshire to reach the Atlantic coast almost to Maine (Green 2005). Populations along the southern shore of Lake Erie in Pennsylvania and Ohio are not continuous with other populations of the species further south. Recent evidence indicates that these populations are related to, and possibly derived from, Canadian populations on the north shore of the lake and are subject to the same threats (Smith 2004). The status in the occupied states is generally secure, with only Vermont and Ontario listing the species at risk.

Eligibility criteria

Native status

[yes] Well-established native taxon of southwestern Ontario.

Taxonomic distinctness

[yes] Fowler’s Toad was formerly considered a subspecies of Woodhouse’s Toad, Anaxyrus woodhousei fowleri. The species occasionally hybridizes with the American Toad (Anaxyrus americanus) and such hybrids occur in Ontario at Long Point (COSEWIC 2010).

Designatable units

The three extant populations of Fowler’s Toads in Canada (Rondeau, Long Point and Niagara) are genetically distinct and morphologically distinguishable (COSEWIC 2010). There is no evidence of any significant, ongoing genetic exchange between these three populations (Smith and Green, 2004), and each population shows little intra- population variation. Genetic distinction between Long Point toads and those from the other two areas, arises in part from an apparent past hybridization with American Toads that occurred before the Long Point toads arrived in Lake Erie (Smith and Green, 2004). There is spatial disjunction among the three populations, but it isn't possible to decide if or how much this disjunction is from natural or anthropogenic causes. Similarly, the genotypes of extirpated populations (e.g., Pelee Island, Point Pelee, Niagara area sub-populations) are unknown so the original phylogeographic pattern along Lake Erie is also uncertain. The three populations occupy the same ecozone. In terms of significance of the genetic separation, again the picture is murky. There is mtDNA sequence variation, but not nuclear DNA sequence variation. The importance of the mtDNA variation is unclear, so it is also unclear whether the loss of one of the populations would constitute a significant loss to the species in Ontario. The populations do not exist in unique ecological settings. The threats to the persistence of Fowler’s Toads at Niagara differ somewhat from those affecting populations at Rondeau Provincial Park and Long Point, but there is overlap in threats among the three areas (COSEWIC 2010). Therefore, the three populations could qualify as separate Designatable Units, or one could also dispute this decision. Regardless of what decision were made, it is likely that the designation of each putative unit or of the whole would be the same, so there would appear to be negligible additional conservation value in assessing two or three separate DUs. Given the conflicting evidence and questionable value, the most parsimonious decision is to treat the three populations as a single DU.

Priority-setting criteria

Recent arrival




Primary criteria (rarity and declines)

  1. Global Rank

    [Not in any category]. G5

  2. Global Decline

    [Not in any category]. No evidence of any widespread decline in the USA

  3. Northeastern North America Ranks

    [Not in any category]. It is ranked S3-S5 in 17 of 19 jurisdictions, and S1 in VT and S2 in ON (~11%) (Appendix 1).

  4. Northeastern North America Decline

    [Not in any category]. There are few declines and none of them appear widespread although the species is not closely tracked (Oldham and Austen 1999).

  5. Ontario Occurrences

    [TH]. There are perhaps 15-16 extant element occurrences in Ontario according to the NHIC Element Occurrence database. These occur generally as small (<500 adults) populations scattered along the Lake Erie shoreline, rarely >500 m inland and grouped in 3 areas (Rondeau Provincial Park, Long Point and the "Niagara" area (i.e., between Rondeau PP and near Niagara along the north shore of Lake Erie) (COSEWIC 2010).

  6. Ontario Decline

    [SC]. The species has disappeared from Pelee Island, PPNP and several small areas along the Erie shore. Overall, it has been lost from at least 10 historic locations (COSEWIC 2010). In areas where threats are less diverse, such as Long Point itself, numbers are declining (D. Green pers. comm. June 1, 2010).

  7. Ontario’s Conservation Responsibility

    [Not in any category]. (Ontario comprises less than 10% of the global range of Fowler’s Toad.)

Secondary criteria (threats and vulnerability)

  1. Population Sustainability

    [EN]. Meets Endangered on the basis of a quantitative analysis based on a PVA model (Vortex) allowing for limited environmental stochasticity, no connectivity between the three populations, limited and variable connectivity among subpopulations, structured age-specific estimates of abundance and loss of habitat from invasive Phragmites a. australis (Common Reed). These analyses estimate a 69% probability of extirpation in 20 years. This is based on the most realistic scenario. A best case scenario yields a 22% probability of extirpation in 20 years (COSEWIC 2010).

  2. Lack of Regulatory Protection for Exploited Wild Populations

    [Not in any category].

  3. Direct Threats

    [EN]. Fowler’s Toads are confined to a narrow band of habitat within a maximum of about 500 m of Lake Erie. This habitat must contain beach, dunes and temporary pools all within close proximity. Sandy beaches are under huge pressure from people conducting myriad forms of recreation, housing and manipulation of sand. The species is considered to be at risk of disappearance or in severe decline at >75% of element occurrences in Ontario due to demonstrated threats, such as: building of large beachfront homes and other urban structures, cleaning, grading and raking of beaches (which removes cover), erection of structures to protect shorelines from erosion, draining of temporary wetlands to remove mosquitoes etc, piling of toxic algae, shells of zebra and quagga mussels on the beaches, vehicle (trucks, ATVs) use of beaches, disturbance or harassment which negatively influences key life-history processes, or results in mortality; fragmentation or isolation of habitat or populations by breakwalls, roads, piers, groynes, parking lots, canals, loss of wetlands, general creation of habitat the toads can not use, replacement of cottages by large, year round homes and higher housing density, displacement by introduced exotic species, particularly Common Reed, increased toxins from agricultural runoff, botulism, spraying of insecticides and herbicides, and various chemicals (e.g., nickel oxide) from increased human presence. Fowler’s Toad populations exhibit large, stochastic fluctuations in population size even from one year to the next. Such fluctuations have been observed in many other species of pond-breeding anurans, especially those with short life expectancies and high early mortality rates (Berven 1995; Green 1997; Pechmann et al. 1991; Green 1997). Stochastic episodes of rise and decline within populations reduce the effective size of those populations relative to the actual number of animals that may be present. This is more important to estimating long-term survivorship than are population maxima or a long-term mean population size (Meffe and Carroll 1994). For example, even when toads manage to linger on in such places (e.g., Crystal Beach) they cannot disperse to or from many sites and when a local extirpation occurs from a stochastic event, the species cannot reinvade.

  4. Specialized Life History or Habitat-use Characteristics

    [EN]. The habitats required by Fowler’s Toads are in general decline, principally because of shoreline development, beach stabilization, vehicular traffic and recreational use at Niagara and the spread of the invasive common reed grass, Phragmites australis australis, at Long Point, Rondeau Provincial Park (COSEWIC 2010). Natural sand beaches along Lake Erie are an increasingly rare and heavily impacted habitat. Open sand beach is ranked S2S3 in Ontario (Bakowsky 1996). Fowler’s Toads are dependent on dynamic habitat changes. Irregular winter storms reduce adult population size, yet the disturbance caused by these storms creates suitable breeding habitat used by the surviving toads (Green 1998). The key problem for these toads is that while winter storms create wonderful dune habitat, in doing so they kill the toads and these new areas of habitat must be invaded from neighbouring populations or subpopulations. But as habitat is lost to development it gets harder for toads to disperse among the areas of suitable habitat. Thus the three populations are now completely isolated from one another and, increasingly, the smaller subpopulations, particularly in the Niagara area, are becoming isolated. Small, isolated populations have poor long term prospects, because their populations tend to be well below Minimum Viable Population (MVP) size (Reed et al. 2003; Traill et al. 2007), and this is exacerbated because the toads undergo extreme fluctuations typical of anurans. These outcomes are well reflected in the PVA model referred to above.

  5. Mortality Trends

    Insufficient Information. Winter storms can cause severe mortality in adult Fowler’s Toads, but these are irregular events, and there is no known "definite trend of elevated mortality" known for this species in Ontario (COSEWIC 2010). Although Fowler’s Toads hybridize with American Toads in Ontario, this hybridization is not thought to be a threat to the former since hybridization is rare and introgression appears to be negligible (Green 1998). Survivorship from egg to adult is ~ 0.0007. Post metamorphic survival rates average about 17% for the first year, then drop to only 4% in the 2nd year, and 2% the 3rd year and 0.01% by the 4th year (COSEWIC 2010). It is a rare toad that survives to 4 years.

COSSARO criteria met

Endangered – [0/3]
Threatened – [1/0]
Special concern – [1/0]


Fowler’s Toad occurs in most US states south and east of the Great Lakes. In Canada, it is confined to less than 30 km of the Lake Erie shoreline, rarely occurring more than 500 m inland. Over the past few decades, it has declined with several populations disappearing. This toad is severely limited by the amount of available habitat. Threats to the species are numerous as its beach habitat is under great and increasing pressure for recreation and housing. Major threats include manipulation of beach habitat, loss of beaches, dunes and temporary ponds for breeding, decreased connectivity among occupied habitats, impact of invasive species especially Common Reed and Zebra Mussels, and increased levels of toxins including pesticides, industrial waste and botulism. A PVA based on a long-term population study indicates that the species has a high probability of extirpation from Ontario within the next 20 years. Therefore, COSSARO has assessed the species' Ontario status as Endangered.

Information sources

Bakowsky, W.D. 1996. Natural Heritage Resources of Ontario: Vegetation Communities of Southern Ontario. Natural Heritage Information Centre, Ontario Ministry of Natural Resources, Peterborough, Ontario. 21 pp.

Berven, K. A. 1995. Population regulation in the wood frog, Rana sylvatica, from three diverse localities. Australian Journal of Ecology 20:385-392.

Green, D.M. 1997b. Population Ecology of Fowler’s Toads, Bufo fowleri, at Long Point, Ontario. World Wildlife Fund, Endangered Species Recovery Fund.

Green, D.M. 1998. Status Report Update on Fowler’s Toad, Bufo fowleri, in Canada. Unpublished report, Amphibian and Reptile Subcommittee, Committee on the Status of Endangered Wildlife in Canada (COSEWIC), Ottawa, Ontario. 18 pp.

Green, D.M. 2005. Bufo fowleri, Fowler’s toad. pp.766-778. In Amphibian Declines: the conservation status of United States species. M.J. Lannoo (ed.). University of California Press, Berkeley.

Meacham, W. R. 1962. Factors affecting secondary intergradation between two allopatric populations in the Bufo woodhousei complex. American Midland Naturalist 67:282-304.

Meffe, G.K., and Carroll, C.R. 1994. Principles of Conservation Biology. Sinauer Associates, Inc. Sunderland, Massachusetts. 600 pp.

Oldham, M.J. 1995. COSSARO V, T, E Species Evaluation Form for Fowlers Toad (Bufo fowleriBufo fowleri). Natural Heritage Information Centre, Ontario Ministry of Natural Resources, Peterborough, Ontario. 7 pp.

Oldham, M.J., and M.J. Austen. 1999. COSSARO V, T, E Species Evaluation Form for Fowlers Toad (Bufo fowleri). Natural Heritage Information Centre, Ontario Ministry of Natural Resources, Peterborough, Ontario. 5 pp.+ 4 appendices.

Pechmann, J. H. K., D. E. Scott, R. D. Semlitsch, J. P. Caldwell, L. J. Vitt, and J. W. Gibbons. 1991. Declining amphibian populations: the problem of separating human impacts from natural fluctuations. Science 253:892-895.

Reed, D.H., J.J. O'Grady, B.W. Brook, J.D. Ballou and R. Frankam. 2003. Estimates of minimum viable population sizes for vertebrates and factors influencing those estimates. Biological Conservation 113:23-34.

Smith, M.A. 2004. Spatial ecology of Bufo fowleri. Ph.D. Thesis. Department of Biology, McGill University, Montreal.

Smith, M.A. and D.M. Green. 2004. Phylogeography of Bufo fowleri at its northern range limit. Molecular Ecology 13:3723-3733.

Traill, L.W., C.J.A. Bradshaw and B.W. Brook. 2007. Minimum viable population size: A meta-analysis of 30 years of published estimates. Biological Conservation 139: 159-166.

Appendix 1

Northeastern North America rank, status and decline


Occurs as a native species in 20 of 29 northeastern jurisdictions SRANK or equivalent information available for 19 of 20 jurisdictions = (95%) S1, S2, SH, or SX in 2 of 19 = (11%)

Part 2: Ontario evaluation using COSEWIC criteria

Regional (Ontario) COSEWIC criteria assessment

Criterion A – declining population

No (N/A); Does not meet criterion A as the size of decline can not be determined.

Criterion B – small distribution and decline or fluctuation

Yes (EN); Meets Endangered B1ab(ii,iii,v)c(iv)+2ab(ii,iii,v)c(iv) as its distribution range is severely fragmented, EO (1,200 km2) and IAO (480 km2) are below thresholds, and area of occupancy, habitat area and quality, and the number of mature individuals are declining. Meets c(iv) because population fluctuates more than an order of magnitude

Criterion C – small population size and decline

No (N/A); Not applicable. Although the minimum number of mature individuals from Rondeau, Long Point and Niagara is below the Endangered threshold (2347), the mean number (4740) is above the Endangered threshold, the decline rates are not calculable, some populations exceed 250 mature individuals, and no populations contains more than 95% of all mature individuals.

Criterion D – very small or restricted

No (EN, TH, or N/A); Does not meet criterion D as the total number of mature individuals exceeds threshold.

Criterion E – quantitative analysis

Yes (EN); Meets Endangered under criterion E as a quantitative analysis based on a PVA model allowing for limited environmental stochasticity, no connectivity between populations, structured age-specific estimates of abundance and loss of habitat from invasive Phragmites reeds estimates a 69% probability of extirpation in 20 years. This is based on the most realistic scenario. A best case scenario yields a 22% probability of extirpation in 20 years.

Rescue effect

No. There is little or no chance of a rescue effect from US populations on the south shore of Lake Erie. The distances between these populations are too great for the toads to traverse and there is no suitable intervening habitat. Information from mitochondrial DNA-sequencing shows that Long Point toads are genetically distinct from other Canadian populations, indicating that there has been no exchange of individuals between these populations for thousands of years. Furthermore, any toad that was swept into open water would tend to be transported by the general surface circulation patterns in Lake Erie from the north shore to the south shore. Thus the US populations are more likely to benefit from a rescue effect from Canada than vice-versa (Smith and Green 2004; COSEWIC 2010).